17. Inheritance

Question 1

DNA - What is it

Answer 1

A large molecule made from bases, and which carries genetic information in the form of genes.

Two strands coiling together to form a double helix. (each strand contains a sequences of bases)

(one molecule)

–>

DNA is tightly coiled + contains large numbers of atoms

Question 2

Chromosomes

Answer 2

Thread-like structures of DNA, carrying genetic information in the form of genes.

A structure containing tightly coiled DNA which is found in the nucleus

Question 3

Gene

Answer 3

A length of DNA that codes for a particular protein.

Defines the genetic information whcih DNA carries.

Question 4

Chromosomes + Genes

Answer 4

Every chromosome PAIR contains different types of genes (coding for different proteins)

In a chromosome pair, both chromsome contain the same gene (but they either have the same or different allele/ version of the gene)

Question 5

Haploid

Answer 5

Single set of chromosomes (a type of nucleus), eg. gamete

23 unpaired chromosomes

Question 6

Diploid

Answer 6

Double set of chromosomes (a type of nucleus), eg. body cells.

23 pairs of chromosomes

Question 7

Sex chromosomes

Answer 7

Two out of 46 chromosomes are sex chromosomes (determine sex of individual)

Either X or Y chromosomes…

Females - XX
Males - XY

Question 8

Sex chromosomes in gametes

Answer 8

The egg cell ALWAYS contains one X chromosome (inherited from mother)

The sperm cell contains EITHER an X chromosome or a Y chromsome (inherited from father)

—> Therefore sex of child is dependent on father (x –> girl, y –> boy)

Question 9

Base sequence in DNA

Answer 9

Each strand contains a sequences of bases.

Base sequence determines genetic code (order of amino acids)

four bases = A, T, C, G

Complementary base pairs –>

A + T
C + G

Question 10

What happens when a gene is expressed?

Answer 10

When a gene is being used in a cell to make a protein, it’s described as being expressed.

Some genes are never used in a cell, other are at different stages.

(eg. insulin production gene is only expressed in pancreas)

Question 11

Stages of protein synthesis

Answer 11

1. A DNA molecule in the nucleus unzips → exposing the bases in a gene.
2. Using the exposed bases of the DNA strand as a template → an mRNA molecule is formed.
3. This happens through the bases in the mRNA molecule pairing with the complementary bases in the DNA. → Therefore the mRNA carries a copy of the gene.
4. The mRNA molecule then moves out of the nucleus and into the cytoplasm.
5. A ribosome reads the mRNA code and then uses the code to join amino acids together to form a protein.

Question 12

Why is mRNA used to transport DNA?

Answer 12

DNA molecules are too large, mRNA only copies the gene needed.

Question 13

mRNA structure vs DNA structure

Answer 13

mRNA = single stranded + base T is replaced by base U

Question 14

Mitosis

Answer 14

A form of nuclear division that gives rise to genetically identical cells.

Question 15

Stages of mitosis

Answer 15

1. Chromsomes duplicate (they look like x’s as they duplicate)
2. Nucleus of the cell divdes
3. Chromosome ‘x’s’ (duplicates) divide into single strands of chromosomes, so each cell has 46 each.

1 cell → 2 cells → 4 cells → 8 cells

Question 16

When does mitosis occur?

Answer 16

Zygote division
Repairing damaged tissues/ replacement of cells

Asexual reproduction

Question 17

Stem cells

Answer 17

An unspecialised cell that can develop into various differentiated cell types.

The cells are still genetically identical in an individual person, but their genes are expressed differently.

Question 18

Where are stem cells found?

Answer 18

Embryos
Umbilical cords
Adult bone marrow

Question 19

Meiosis

Answer 19

A type of nuclear division that gives rise to cells that are genetically different.

Involved in production of gametes (in all organisms that sexually reproduce, includign flowering plants)

Question 20

Stages of meiosis

Answer 20

1. A diploid cell duplicates its chromosomes.
2. Divides into two diploid cells.
3. Then each diploid cell halves, forming in total four haploid cells.

(all haploid cells are genetically different from each other)

Question 21

How does variation occur during meiosis?

Answer 21

As the chromosomes duplicate (look like x’s), the chromosomes in a pair swap parts with each other (aka they ‘crossover’)

The crossed over chromosomes then divide to form haploid cells.

eg.

(X, X) (Chromosome pair in duplication process)

(X, X) (crossover)

(X) (X) (chromosome pair divided into 2 haploids)

(l) (l) (duplicated chromosome divided into 1 unpaired chromosome –> haploid)

Question 22

Inheritance (def + types of inheritance)

Answer 22

The transmission of genetic information from generation to generation.

Genotypic

Sex
Blood group
Natural hair colour

Phenotypic

Language

Both

Height + Body mass

Question 23

Homozygous

Answer 23

Two identical alleles in a chromosome pair

Question 24

Heterozygous

Answer 24

Two different alleles in the chromosome pair.

Question 25

Allele

Answer 25

An alternative version of a gene

Question 26

Genotype

Answer 26

An organism’s genotype is all the alleles present in that organism. (The genetic make-up in terms of the versions of genes in contains)

Question 27

Phenotype

Answer 27

The observable features of an organism. (Some aspects of the phenotype is a result of teh genotype, eg. eye colour)

Question 28

Homozygous green seed * Homozygous green seed (pea plant example)

Answer 28

--> Only one possibility of genes can be inherited. (green seeds because both parents only have the green seed gene)

Question 29

Heterozygous seed colour * Homozygous green seed

Answer 29

The offspring will inherit an allele for green seeds from their homozygous parent. However, they can inherit either the green or yellow seed allele from their heterozygous parent.

Question 30

Dominant

Answer 30

When present the dominant allele will always be expressed.

Question 31

Recessive

Answer 31

When present the recessive allele will only be expressed if the dominant allele is not present.

Question 32

Dominant/ recessive combinations

Answer 32

Dominant + Recessive = Dominant Recessive + Dominant = Dominant Dominant + Dominant = Dominant Recessive + Recessive = Recessive

Question 33

Genetic symbols

Answer 33

A dominant allele is shown by an uppercase letter A recessive allele for the same gene is shown by the same letter but in lowercase.

Question 34

Codominance

Answer 34

In some cases, two different alleles for a characteristic influence the phenotype to the same extent. Eg. Blood group

Question 35

Blood group - Codominance

Answer 35

People can belong to blood group A, B, AB or O. The gene involved is shown by an uppercase letter, I and then a subscript uppercase letter. Possible genotypes... I^A + I^A --> A I^A + I^O --> A I^B + I^B --> B I^B + I^O --> B I^A + I^B --> AB I^O + I^O --> O ------------------------- Alleles I^A and I^B are both dominant to allele I^O Alleles I^A and I^B are codominant to each other – a person inheriting both alleles is blood group AB (not A or B).

Question 36

Sex linkage

Answer 36

Sex in humans is determined by the two sex chromosomes, X and Y. Males are XY and females are XX. Sex chromosomes don’t just determine the sex of an organism. --> They also carry certain genes which determine other characteristics. Eg. Haemophilia + red-green colour blindness are recessibe genes carried on X chromosome.

Question 37

Y vs X chromosome

Answer 37

The X chromosome carries many more genes than the Y chromosome because it is much longer. Many genes found on the X chromosome are not found on the Y chromosome.

Question 38

Why is it more common for men to be colourblind / haemophiliacs

Answer 38

The gene is recessive and carried on the X chrosomes Women need two recessive genes (X^r + X^r) However, men only need one recessive gene (X^r + Y) --> Making it more likely.